The present invention relates to a method for preparing perfluoropolyether oligomers, and, more particularly, to a method of electrochemical coupling of perfluorocarboxylic acid salts or mixtures thereof in aqueous methanol under controlled conditions to produce perfluoropolyether oligomers.
Perfluoropolyethers possess excellent chemical and thermal stability which ensures a wide field of application for these compounds. They are useful as heat transfer media, sealing liquids, lubricants under extreme chemical conditions, additives for oils and greases, turbine propellents and hydraulic liquids. They exhibit low dielectric constants, high breakdown voltages and low loss factors in alternating fields, which makes them particularly suited for applications in the electrical area. In the electronics industry, for example, perfluoropolyethers are particularly useful as vapor phase soldering fluids. They also find applications as convection cooling liquids in transformers or similar devices. Their excellent dissolving power for oxygen and carbon dioxide enables them to be used as oxygen conveyers in heart-lung machines and also directly as blood substitutes in living organisms. These products also find applications in many spheres of nuclear and chemical engineering. Because of their outstanding chemical resistance they are superior to the propylene oxide fluids that hitherto dominated these application fields, especially at temperatures above 200.degree. C.
The compositions prepared by the process of the present invention are described in U.S. Pat. No. 3,214,478, which according to the reference the compositions are obtained by exposing low molecular weight perfluoropolyethers to ultraviolet irradiation. The reaction times ranging from 32 hours to seven days may be unsuitable for economic commercial manufacture. Further, U.S. Pat. No. 4,052,277 describes a photochemical process for preparing perfluorinated polyethers from perfluorocarbonyl compounds, which require exposure times from 10 to 20 hours and result in carbon monoxide being produced.
Electrolytic condensation or electrochemical coupling of carboxylic acids, also known as the Kolbe process, offers an economical route in the preparation process. For example, U.S. Pat. No. 2,760,926 describes hydrocarbon aliphatic ethers prepared by electrolysis of salts of aliphatic carboxylic acids under anhydrous or substantially anhydrous conditions. Lower monohydric and dihydric alcohols, such as methanol, are used as solvents. Further, the application of the Kolbe process to a continuous electrolytic cell with less than 20% molar degree of neutralization of the carboxylic acid is described in U.S. Pat. No. 3,787,299. Adverse gas-filling effects of the Kolbe process are diminished by limiting the distance between the electrodes to synthesize difunctional compounds from substituted but not alpha-substituted alkanoic acids. The compounds produced are useful as intermediates in manufacturing polyamides or polyesters and in manufacturing special plasticizers or ester oils.
Finally, Japanese Kokai Patent No. SHO 61-21610 (1986) describes 37 to 64% yields of the objective perfluoropolyether by electrolyzing a perfluoroalkoxycarboxylic acid or its soluble salt, such as sodium, potassium, lithium, ammonium or magnesium, in a polyhydric alcohol solvent such as ethylene glycol. The reference states that if methanol or water-containing methanol are employed as solvents, it is virtually impossible to synthesize the objective perfluoropolyether. The compounds produced are used to develop synthetic blood.
It has been discovered that aqueous methanol can be employed as a solvent in the process of the present invention. During electrolysis, however, gel formation around the electrodes is a major obstacle for commercial scale electrolysis. It has further been discovered that maintaining the electrolysis under carefully controlled conditions, the gel formation is substantially reduced to give a practical, commercial, feasible process. The electrochemical coupling of the salts of carboxylic acids derived from perfluoropolyethers or mixtures thereof in aqueous methanol under controlled conditions constitutes an economical route in the process of the present invention.